Quaternary ammonium salts carrying two hydrophobic long chain hydrocarbon moieties have found broad use as fabric softener actives. Quaternary ammonium salts of alkanolamines esterified with on average two fatty acid moieties per molecule, commonly referred to as ester quats, have largely replaced earlier alkyl quaternary ammonium compounds because of their biodegradability.
For use in rinse cycle softener products, a softener active composition has to meet several and sometimes conflicting requirements:                High softening performance in terms of soft touch and fabric rewettability,        good storage stability in aqueous dispersion with little change in dispersion viscosity, and        convenient handling and processing in a liquid state.        
The ester quats which have found the broadest technical use and which today set the standard for softening performance are methyltriethanolammonium methylsulphate fatty acid diesters and dimethyldiethanolammonium chloride fatty acid diesters. However, aqueous dispersions of these fabric softener actives have limited stability and extended storage of such aqueous dispersions at temperatures in excess of 40° C. will usually lead to an unacceptable rise in dispersion viscosity or to settling of the softener active. Furthermore, these fabric softener actives cannot be handled and processed to aqueous dispersions without the addition of a solvent because of their high melting points and melt viscosities and the limited thermal and hydrolytic stability of the fabric softener actives. Therefore, they are usually delivered and processed with a content of 5 to 15% by weight ethanol or isopropanol, which requires additional precautions due to the volatility and flammability of the solvent.
EP 0 293 955 A2 and EP 0 302 567 A2 disclose aqueous fabric softener dispersions having high storage stability and little change in viscosity during storage and a method for preparing such dispersions. These compositions contain a bis-(2-hydroxypropyl)-dialkylammonium salt fatty acid diester as the fabric softener active in the form of submicrometer particles. However, preparation of these dispersions requires processing the fabric softener active mixed with from 5 to 50% by weight of a C1-C4 monohydric alcohol. In the examples, bis-(2-hydroxypropyl)-dimethylammonium chloride palmitic acid diester is used as the fabric softener active and isopropanol is used as the solvent.
DE 24 30 140 C3 discloses bis-(2-hydroxypropyl)-dialkyl-ammonium salt fatty acid diesters for providing liquid fabric softener actives. Example 2 discloses the preparation of a bis-(2-hydroxypropyl)-dimethylammonium methylsulphate fatty acid diester by reacting a bis-(2-hydroxypropyl)-methylamine fatty acid diester of a fatty acid having an average chain length of 19 to 20 carbon atoms and comprising 90% by weight unsaturated fatty acid moieties with dimethyl sulphate in a molar ratio of 1:1.
EP 1 018 541 A1 discloses clear fabric softener compositions comprising an ester quat and an alkoxylated phenol or branched C3-C6 alcohol solvent. Example 6 discloses a composition containing a bis-(2-hydroxypropyl)-dimethylammonium methylsulphate fatty acid ester having a molar ratio of fatty acid moieties to amine moieties of 1.8 derived from a fatty acid having an average chain length of 18 carbon atoms and an iodine value of about 150. The ester quat active is processed with addition of 10% by weight isopropanol when making this composition, as disclosed in paragraph [0026].
WO 00/06678 discloses incompletely esterified ester quats of branched chain alkanolamines, which are claimed to have low melting points and high hydrolytic stability, and proposes to leave on average one hydroxyl group of the alkanolamine non-esterified. Example 50 discloses a bis-(2-hydroxypropyl)-dimethylammonium methylsulphate fatty acid ester made by quaternising a bis-(2-hydroxypropyl)-methylamine fatty acid ester having a molar ratio of fatty acid moieties to amine moieties of 1.26 derived from a fatty acid having a chain length of 12 to 14 carbon atoms.
DE 36 08 093 A1 discloses concentrated aqueous fabric softener compositions comprising an ester quat with two acyl groups, a fatty acid or an alkali salt thereof in an amount of 1/70 to ⅓ of the amount of the ester quat and a solvent combination of water, glycerol and an additional organic solvent in a total amount of ⅙ to twice the amount of the ester quat. Example 4 discloses a composition containing 45% by weight bis-(2-hydroxypropyl)-dimethylammonium methylsulphate oleic acid diester, 1% by weight tallow fatty acid sodium salt, 11.5% by weight water, 11.5% by weight glycerol, 17.5% by weight 2-propanol, 6% by weight propylene glycol and 3% by weight dipropylene glycol.
The ester quat actives disclosed in DE 24 30 140 C3, EP 1 018 541 A1 and WO 00/06678 have low melting points, but provide insufficient softening performance due to the high degree of unsaturation of the fatty acid moieties or the high content of monoester quat component. On the other hand, similar ester quats derived from bis-(2-hydroxypropyl)-methylamine with a low content of monoester quat, made from fatty acids with a low degree of unsaturation, as the one disclosed in EP 302 567 A2, provide the required softening performance, but show high melting points and melt viscosities and therefore require addition of a solvent for handling and processing.
Therefore, there is still a need for fabric softener actives which can be handled and processed without a solvent and without compromising storage stability in aqueous dispersion with little change in dispersion viscosity.